As shown in FIGS. 7 and 8, a conventional rotary compressor includes a closed container 1, an electric motor (not shown) accommodated within the closed container 1, and a compression mechanism A similarly accommodated within the closed container 1 and connected to the electric motor via a shaft 4. An oil sump is formed in the closed container 1 at a bottom portion thereof. The compression mechanism A includes a cylinder 5 having a radially extending vane groove 10 defined therein, a main bearing 7 and an auxiliary bearing 8 secured respectively to opposite end surfaces of the cylinder 5 to define a cylinder chamber 6, a shaft 4 having an eccentric portion 41 formed between the main bearing 7 and the auxiliary bearing 8, a piston 9 mounted on the eccentric portion 41 of the shaft 4, and a vane 11 loosely inserted in the vane groove 10 for a reciprocating motion thereof. The vane 11 has a distal end 11A hingedly connected to a joint 9A formed in the piston 9 to partition the cylinder chamber 6 into a suction chamber 12 and a compression chamber 13.
Rotation of the shaft 4 is followed by an orbital motion of the piston 9 and a reciprocating motion of the vane 11, both of which in turn cause a change in volume of the suction chamber 12 and a change in volume of the compression chamber 13. Such volumetric changes compress a working refrigerant, inhaled into the suction chamber 12 through a suction port 17, into a high-temperature and high-pressure refrigerant, which is discharged from the compression chamber 13 into the closed container 1 through a discharge port 18 and a discharge muffler chamber 19. At the same time, oil stored in the oil sump is sucked by an oil pump mounted on a lower end of the shaft 4 and passes through a through-hole defined in the shaft 4. The oil is then supplied to and lubricates sliding surfaces in the compression mechanism A such as, for example, those between the eccentric portion 41 of the shaft 4 and an inner peripheral surface 9B of the piston 9 and those between an outer peripheral surface of the piston 9 and an inner peripheral surface of the cylinder 5 (see, for example, Patent Document 1).
In the above-described conventional rotary compressor, as shown in FIG. 9, a viscous force of the oil acting between the eccentric portion 41 of the shaft 4 and the inner peripheral surface 9B of the piston 9 generates a rotational moment about a center of the eccentric portion 41 of the shaft 4. This rotational moment acts on the piston 9 in a direction of rotation of the shaft 4 and is supported by the distal end 11A of the vane 11. Accordingly, frictional resistance forces are exerted as reaction forces of this support force on contact points 201 and 202 between the vane 11 and the vane groove 10, thus increasing a sliding loss that is generated by the reciprocating motion of the vane 11 within the vane groove 10. In the rotary compressor of this kind, in order to reduce the sliding loss to reduce an input loss, it is preferable to minimize the viscous force of the oil acting between the eccentric portion 41 of the shaft 4 and the inner peripheral surface 9B of the piston 9 by reducing areas of the sliding surfaces between the eccentric portion 41 of the shaft 4 and the inner peripheral surface 9B of the piston 9 or by reducing a sliding speed of one of the eccentric portion 41 of the shaft 4 and the inner peripheral surface 9B of the piston 9 relative to the other.